1. Field of the Invention
This invention relates to mutants of plant pathogenic fungi, methods for their preparation, bioherbicidal compositions thereof and methods of using the mutants for weed control. The fungal mutants of the invention are agriculturally and environmentally acceptable and safe due to their inability to survive over prolonged periods of time.
2. Description of the Background
A variety of non-selective broad spectrum chemical herbicides are known and are utilized in situations requiring the control of various species of weeds growing simultaneously in a field (e.g., U. S. Pat. No. 3,503,732). Selective chemical herbicides are also known and are utilized when the eradication of a limited number of weeds is desired (e.g., U. S. Pat. No. 3,457,063).
In areas where chemical herbicides cannot be safely or economically utilized, the biological control of weeds is an alternative possibility. Approaches which have been used in the past for developing biological weed control agents have been of two different types.
The first relies on the isolation of native or exotic pathogens or pests which are highly effective and at the same time specific to a particular weed. This need for both specificity and effectiveness has severely limited the number of organisms that can be successfully used to control weeds (e.g., U.S. Pat. Nos. 3,999,973 and 3,849,104).
An alternative approach is the use of mutation and selection to isolate highly lethal but non-specific pathogens or pests which are limited to the area of application or to a target species (e.g., U.S. Pat. No. 4,489,161). In the '161 patent, disease causing organisms are mutated to obtain variant strains useful to reduce the deleterious effect of the plant pathogenic microorganism fusarium wilt on chrysanthemums. Sands, D. C., and Rovira, A. D., "Modifying the Virulence and Host Range of Weed Pathogens, p. 305, Proceedings Third Int. Conf. on Plant Pathogenic Bacteria, Geesteramis, H. P., Ed., Centre Agricultural Publications Documents, Wageningen, The Netherlands (by one of the present inventors) disclosed the preparation of a mutant Pseudomonas bacterium which is harmless to the single type of desired plants (tomato plants) but kills weeds such as nightshade. Charudattan, R., "The Use of Natural and Genetically Altered Strains of Pathogens for Weed Control", pp. 347-372, Biological Control in Agricultural IPM Systems, Academic Press, Inc., N.Y., (1985), discussed generally the use of genetically altered organisms for biological control of weeds.
Accordingly, the provision of a plant-sparing fungal bioherbicide useful for the control of weeds which does not persist in the environment for a prolonged period of time would be a great advance in the area of weed control.